Whatever their domestic achievements, the legacies of American wartime leaders are largely defined by their wars — think Lyndon Johnson and Vietnam or George W. Bush and Iraq. While U.S. presidents tend to enjoy a patriotic bump at war’s outset, long, costly military engagements nearly always drag down their popularity over time.

The more interesting question, according to economist Benny Geys of Berlin’s Social Science Research Center, is what, exactly, it is about war that tends to make Americans turn against their presidents. Public-opinion researchers have traditionally used combat deaths to measure the amount of “pain” a war inflicts on society, but this doesn’t quite fit: The unpopularity of the Iraq War during the Bush administration matched — and at some points even exceeded — opposition to the Vietnam War, despite the fact that more than 58,000 U.S. troops were killed in Vietnam versus fewer than 5,000 to date in Iraq.

For this reason, Geys views treasure — not blood — as the more relevant indicator. In the case of the Korean War, Geys shows that defense costs alone accounted for a nearly 5-point dip in Harry Truman’s popularity.

The effect is lower, though still statistically significant, in later conflicts, which Geys argues could be a result of the declining cost of war relative to the size of the U.S. economy. In 1952, the Korean War accounted for 4.2 percent of U.S. GDP. In 2008, the most costly year of the Iraq War, it was just 1 percent of GDP.

Geys doesn’t consider one important but difficult-to-quantify factor: whether the public thinks a war is worth fighting. For example, approval ratings for Franklin D. Roosevelt remained high throughout World War II, despite enormous human and financial costs, because it was almost universally seen as a necessary sacrifice.

The relative expenditure of treasure and blood is much lower today — perhaps one reason why the United States is approaching nearly a decade of continuous war in Afghanistan. But if Barack Obama’s poll numbers continue to drop, he might realize that there’s only so long the public will support a war — even at a discount.

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Uri Friedman is deputy managing editor at Foreign Policy. Before joining FP, he reported for the Christian Science Monitor, worked on corporate strategy for Atlantic Media, helped launch the Atlantic Wire, and covered international affairs for the site. A proud native of Philadelphia, Pennsylvania, he studied European history at the University of Pennsylvania and has lived in Barcelona, Spain and Geneva, Switzerland.